Metal coating carbon substrates



United States Patent 3,547,692 METAL COATING CARBON SUBSTRATES James S.Hill, Cranford, N.J., assignor to Engelhard Minerals & ChemicalsCorporation, Newark, N.J., a corporation of Delaware No Drawing. FiledOct. 17, 1968, Ser. No. 768,499 Int. Cl. B44d 1/18 US. Cl. 117-217 4Claims ABSTRACT OF THE DISCLOSURE carbonaceous materials are providedwith an adherent solderable metal coating by initially depositing anickel coating on the carbon surface by immersion plating, treating thecoated substrate at elevated temperature until substantiallynon-metallic in appearance, replating the treated substrate with asecond metal and heating to diffusion bond the second metal to theinitially deposited coating.

This invention relates to coatings on carbon and, particularly, to aprocess for coating a carbonaceous surface with an adherent metalcoating having good solderability.

Carbon, particularly the graphite form of carbon, has utility in thefabrication of electrical contacts, e.g. for use as contact points inmotors and generators and as semiconductor contact points. In order toeffectively employ graphite in electrical contact applications, it isnecessary to provide a firm, adherent and solderable metallic film tothe contact element. Graphite is relatively porous and readilypenetrated by many liquids and gases. The high porosity and poor surfacehardness of graphite render it diflicult to provide an adherent metalliccoating to the graphite surface and to which electrical leads can besoldered without causing spalling and flaking of the metallic coating.

In accordance with the present invention, an improved adherent metalcoating is applied to a carbon substrate by electrolessly depositing athin layer of nickel on the carbon substrate; heating the coatedsubstrate to a temperature above about 800 C. until substantiallynon-metallic in appearance; depositing a second metal coating on thetreated substrate and heating the coated substrate at a temperature offrom about 500 C. to about 700 C. for a time sufficient to bond thesecond metal to the treated substrate.

In practicing the process of the present invention, carbonaceousmaterial, for example graphite contact points which are preferablyfabricated of pyrolytic graphite, are first brushed free of loosesurface material and then coated with a thin deposit of nickel by thechemical reduction method. This chemical reduction method, more commonlyknown as electroless plating, employs a series of specific steps whichinclude sensitizing the substrate surface with a reducing agent in theform of a stannous salt bath, rinsing off the stannous solution,activating the surface in a noble metal salt solution, and thenelectroless plating. Such techniques of electroless plating are by nowwell known in the art, and need not be further described in detail. Forthe purpose of depositing nickel on the activated and sensitizedsurface, well-known electroless nickel plating baths, e.g. baths of thetype disclosed in Brenner and Reddell, US. Pat. No. 2,532,283, or inGutzeit et al., US. Pat. Nos. 2,658,841 and 2,658,842, may be employedin the practice of the present invention.

I have found that graphite pellets, nickel coated by the electrolessprocess, cannot be fitted with contact wires by soldering. Attempts tolead solder a copper Wire to the nicked coated pellets results in severespalling and com- 3,547,692 Patented Dec. 15, 1970 plete lack ofadherence of the copper wire to the substrate.

In order to obtain readily solderable surfaces on graphite, thenickel-coated graphite substrate is first heated to a temperature of atleast 800 C. and to 1100 0., preferably 900 C. to 950 C. in anon-oxidizing atmosphere until the metallic appearance of the coatedgraphite substantially disappears and the surface become black in color,followed by cooling in non-oxidizing atmosphere to ambient temperature.Generally the heat treatment is effected for a period of from 0.5 toabout 2 hours and the loss of metallic apperance probably indicates theformation at least in part of nickel carbide in the surface layer.

As a second step in the process of the present invention, the heatedgraphite substrate is reactivated and sensitized, and a second metalliclayer deposited electrolessly thereon employing well-known electrolessplating solutions. As the second metallic layer, any metal which willdilfusion bond with the nickel and/ or nickel carbide layer alreadypresent on the substrate surface can be used, e.g. cooper, nickel,chromium, iron, cobalt and the like. Preferably, the second metal iscopper or nickel. After depositing the second metal on the substrate,the coated substrate is heated to a temperature of about 500 C. to about700 C., suflicient to cause diffusion bonding of the second metalcoating with the first coating, but below the temperature at whichsubstantial formation of any metallic carbide will occur.

Graphite substrates metallized as described herein could be readilyfitted with soldered wire leads without spalling of the surface coating.

EXAMPLE 1 Graphite pellets of 0.5 diameter and .05" thickness weresensitized by immersion in a stannous chloride activator solutionconsisting of Stannous chloride-10 gm. Concentrated hydrochloric acid-40ml. Distilled water1 liter.

for a period of two minutes.

The pellets were then washed with water and immersed for about twominutes in a sensitizing solution consisting of PdCl l gm. Concentratedhydrochloric acid-l ml. Distilled water-1 liter TIhe sensitized pelletswere washed thoroughly with distilled water and then nickel-plated bythe electroless method at 75 C. for one hour in a proprietary nickelplating solution prepared by mixing 25 0 ml. Enplate 410A and ml.Enplate 41013 (products of the Enthone Corp., West Haven, Conn.)diluting with 4 00 ml. water and adjusted to a pH of 4.5 by addition of2 M NaOH, then filtering and adding 1100 ml. water.

At the end of one hour, at which time approximately 50 mg. Ni/cm. hadbeen deposited, the pellets were removed from the plating solution,washed with distilled water and dried.

Attempts to lead-solder a copper wire to the nickelcoated pelletsresulted in spalling.

EXAMPLE 2.

The procedure of Example 1 was repeated, except that the plated pelletsafter drying were placed in an aluminum boat containing alundum sand toprevent the pellets from sticking together. The pellets were then heatedin a hydrogen atmosphere at 930 C. for /2 hour, followed by cooling in Hto ambient temperature. The heated pellets EXAMPLE 3 Pyrolytic graphitepellets were activated and sensitized by treatment with stannouschloride and palladium chloride solutions as described in Example 1.

The treated pellets were washed thoroughly with distilled water and thennickel plated by the electroless method at 85 C. by immersion for onehour in a solution consisting of NHHZPO'Z. gm. Sodium acetate-3 gm.Distilled waterl liter The pellets were then removed from the platingsolution, washed with distilled water, dried, and then heated in ahydrogen atmosphere at 930 C. for /2 hour followed by cooling to ambienttemperature in a hydrogen atmos phere.

The pellets, now black in appearance, were again activated andsensitized as before and subsequently immersed for one: hour in aplating solution at 95 C. consisting of NiSC -6H O17 gm. NaH PO -H O-24Sodium acetate41 gm. Acetic acid30 gm. Distilled waiter1 liter.

After remetallizing the pellets were heated to 600 C. followed bycooling in hydrogen.

A copper wire was readily lead soldered to the coated pellets of thisexample Without spalling.

EXAMPLE 4 Pyrolytic graphite pellets were activated and sensitized inaccordance with the procedures outlined above. Then the treated pelletswere washed with distilled water and metallized by immersion for onehour at 85 C. in the nickel chloride plating solution of Example 3. Nextthe metallized pellets were washed with distilled water, dried andheated for /2 hour in hydrogen at 930 C. followed by cooling to roomtemperature in a hydrogen atmosphere.

4 The pellets were reactivated and sensitized by the procedurepreviously described and subsequently immersed for /2 hour in anelectroless copper plating bath consisting of Potassium sodiumtartrate79 gm. Copper sulfate28 gm.

Sodium hydroxide-20 gm. Sodium carbonate32 gm. Distilled waterl.5 litersFormaldehyde10 cc.

After metallizing with copper, the pellets were heated to 600 C. in ahydrogen atmosphere followed by cooling. The pellets retained theirbright copper metallic appearance.

The copper coated pellets were found to be readily solderable.

What is claimed is:

1. A method for providing a solderable metal coating on a carbonaceoussubstrate which method comprises depositing a nickel film on thesubstrate by plating from an electroless nickel plating solution,treating the coated substrate in a non-oxidizing atmosphere at atemperature between about 800 C. and about 1100 C. for a period of from0.5 to about 2 hours until substantially non-metallie in appearance,cooling the coated substrate in a nonoxidizing atmosphere to ambienttemperature, depositing a second metallic film on said treated substrateby electroless plating and heating the thus coated substrate at atemperature of about 500 C. to about 700 C. to cause diffusion bondingof said second metal coating with the first formed coating.

2. The method of claim 1 wherein said second metallic film is composedof nickel.

3. The method of claim 1 wherein said second metallic film is composed.of copper.

4. The method of claim 1 wherein the carbonaceous substrate is pyrolyticcarbon.

References Cited UNITED STATES PATENTS 2,876,139 3/1959 Flowers 117-71UX3,294,572 12/1966 Piccione et al 11747 3,309,231 3/1967 Hess 1l7228XFOREIGN PATENTS 1,167,150 4/1964 Germany 117-228 ALFRED L. LEAVITT,Primary Examiner c. K. -WEIEFENBACH, Assistant Examiner US. Cl. X.R.

